The science of pharmaceutical chemistry has progressively provided more and more specific and potent drugs for the treatment and prevention of illnesses. However, until quite recently, there has been no means to direct a drug to the specific part of the body where it is needed. Thus, although it is often possible to treat a patient with a drug which has the specific effect which is needed, and no other effect on the body, it is still necessary to administer a whole-body dose. On the other hand, if it were possible to direct a drug to the organ, tissue or even cell in need of the treatment, it would often be possible to administer an extremely small total dose, since the drug would concentrate itself where it is needed. The advantage in safety to the patient and economy of the drug is obvious.
The present invention provides immunoconjugates which utilize derivatized trityl groups to generate a genus of immunoconjugates which provide for release of therapeutic agents at various degrees of acidity or alkalinity. The flexibility thus afforded by the ability to preselect the pH range at which the therapeutic agent will be released allows the clinician to select an appropriate immunoconjugate based on the known physiological differences between tissues in need of delivery of a therapeutic agent, the known antigenic specificity of literally hundreds of monoclonal antibodies which are now available, and various therapeutic agents amenable to use in the immunoconjugates of the present invention.
The present invention provides immunoconjugates useful in numerous therapeutic areas, but is especially well suited to deliver oncolytics. The acidity (pH) of tumor tissues appears to be lower than that of normal tissues. Studies conducted more than half a century ago showed that malignant tumors metabolize carbohydrates mainly by anaerobic glycolysis, even under aerobic conditions. Warburg et al., Biochem. F., 152:309 (1924). The oxidation of glucose stops at the stage of glucose oxidation to pyruvic acid, followed by reduction to lactic acid. Boxer and Devlin, Science, 134:1495 (1961). Most of this lactic acid is either removed or buffered by surrounding extracellular fluid, but some of it accumulates extracellularly. This results in a lower pH within the tumor than in normal tissues. Elevation of the blood-sugar by intravenous infusion of glucose should accelerate anaerobic metabolism resulting in even more lactic acid in the tumor, and this should further increase the pH difference between tumors and normal tissues.
Following Warburg's studies, there have been several reports of lower pH in tumors of both experimental animals [See, e.g., Voegtlin, et al., National Institutes of Health Bulletin, 164:1 (1935); Hahler and Robertson, Journal of the National Cancer Institute, 3:495 (1943)] and human patients. Naeslund, Acta Soc. Med. Upsal., 60:150 (1955); Pampus, Acta Neurochir., 11:305 (1963).
Meyer et al., in Cancer Research, 8:513 (1948), reported that the pH of malignant human tumors is lower than in normal tissues. In twelve out of fourteen cases, where both normal and neoplastic tissues from the same patients could be studied in vivo, there was a difference in pH which averaged 0.49 and ranged from 0.17 to 1.15.
Ashby, [The Lancet, Aug. 6, p. 312 (1966)], found that the mean pH of malignant tumors from nine patients was 6.8 (ranging between 6.6 and 6.9). Raising of the blood sugar by intravenous infusion of dextrose further decreased the tumor pH to a mean of 6.5 (range 6.3-6.8).
Van Den Berg, et al., European Journal of Cancer and Clinical Oncology, 18:457 (1982), showed that the pH of twenty-two human mammary carcinomas was 7.29 (.+-.0.05, SEM), as compared to 7.63 (.+-.0.03, SEM) in human subcutis, and observed similar differences in rat tumors. The differences between pH in tumors and normal tissues were highly statistically significant, although they were lower than those reported in the studies discussed above.
Thistlethwaite, et al., Int. J. Radiation Oncology Biol. Phys. 11:1647 (1985), showed, likewise, that the pH of human tumors as measured by readings on fourteen tumors was below the physiological level with an average of 6.81.+-.0.09 (SEM). They speculated that the reported therapeutic effectiveness of hyperthermia depends on the lower extracellular pH of tumors as compared to normal tissues.
Trouet et al., U.S. Pat. No. 4,376,765, describe drug compounds composed of a protein macromolecule (carrier) linked via a peptide chain ("spacer arm") to an amino function of a drug. The carrier facilitates endocytic take-up by target cells so that the spacer arm may be cleaved within the cell. Recently, attention has been directed to developing antibody drug conjugates which release a drug within a tumor cell once the conjugate has crossed the cell membrane and encountered acidic pH (3.5-5.5) within the cell. U.S. Pat. No. 4,569,789 issued to Blattler, et al., describes chemical formation of conjugates using crosslinking structures which can link amino-group substances such as chemotherapeutic drugs to the sulfhydryl portion of a compound such as an antibody reactive with tumor cell surface antigens capable of crossing the tumor cell membrane. One limitation of such a method of forming conjugates is that the antibody must contain a sulfhydryl group. This reduces the number of possible drug-antibody conjugates which may be formed using such procedures.
U.S. Pat. No. 5,084,560 describes peptide-based "linkers" which release at lower pH values and describes studies wherein the lower pH values of tumors were measured.
European Patent Application 0 424 819 A1 describes the preparation of trityl derivatives and describes their use as protecting agents for the reversible modification of a variety of natural products, biopolymers and the like.